At the Museum of Natural History’s Rose Center for Earth and Space, a group of teenagers on a day-camp field trip eye the scales, where they can see their weight on different planets and other celestial bodies.

Melissa Mann, 12, skips from one scale to another — she’d weigh 4,200 pounds on the sun and .049 pounds on a giant red star.

“I’d weigh nothing on the moon,” says Anais Rodriguez, a 14-year-old girl from The Bronx.

As participants in the Bronx Nutrition and Fitness Initiative (B’N Fit) six-week summer camp, these kids are used to scales. It’s just one part of a weight-loss program for obese adolescents. They’re also getting used to healthier meals, keeping food diaries, running a mile in the morning and living more actively.

The Mosholu Montefiore Community Center and the Montefiore Medical Center’s Children’s Hospital founded B’N Fit in 2005 “because of the high rate of obesity in The Bronx,” says Dr. Jessica Rieder, director of B’N Fit. “We felt we weren’t offering anything comprehensive for the kids.”

Upon signing up, each teen gets a complete medical evaluation, a social worker meets with his or her family and a nutritionist looks at diet and activity.

“It’s the beginning of their taking care of themselves,” says Rieder.

After breakfast at home, the group meets at the community center to take a long bus ride out of The Bronx to someplace such as Jones Beach or Bear Mountain. They also visit the Green Chimneys Farm in Brewster, NY, where the kids learn to cook with basil from the gardens, and Dorney Park in Allentown, Pa., for the rides.

At their destination, they usually stretch, do jumping jacks and other exercises, play a game such as kickball or capture the flag, and run a mile.

“I can do a little more than a mile,” says Simon Darius, 14. “You can jog or power walk, but you have to finish.”

They swim after lunch.

“I used to think exercise was boring,” says Melissa. “It’s not boring. We have fun with it.”

The kids bring their lunches, which B’N Fit nutritionist Rachel Raman inspects, along with food journals, keeping an eye out for chips, pretzels, white bread, soda and other unhealthy food.

“Sometime we joke around and we say, ‘Hide the Twinkies,’ ” says Melissa.

If Raman does find something unhealthy, she quietly pulls the teen aside.

“She’ll talk to you about it,” says Melissa. “You won’t get in trouble or anything.”

By the end of the six weeks, Melissa shed 12.8 pounds. “I feel more active, and everyone’s telling me I look really slim,” says the teen, who promises she’ll run on the track and dance to whatever’s playing on her TV or radio. “I feel proud of myself.”

Simon, who lost 3.2 pounds, also plans to continue his healthy, more active regime, which means running around the park with his cousins instead of just hanging out at home with them. He also plans to brown-bag a healthy lunch for school.

“I want to lose weight,” says Simon. “That’s pretty much why I’m there. I don’t want to be big or skinny, just a regular person.”